A known scroll compressor, or pump, 10 is shown in FIG. 3 and described in more detail in the present applicant's earlier application WO2011/135324. The pump shown in FIG. 3 has an inverted scroll configuration. The pump 10 comprises a pump housing 12 and a drive shaft 14 having an eccentric shaft portion 16. The shaft 14 is driven by a motor 18 and the eccentric shaft portion is connected to an orbiting scroll 20 so that during use rotation of the shaft imparts an orbiting motion to the orbiting scroll relative to a fixed scroll 22 for pumping fluid along a fluid flow path between a pump inlet 24 and pump outlet 26 of the compressor. The fixed scroll is shown generally on the left and the orbiting scroll is shown generally on the right. The fixed scroll comprises an opening 28 through which the shaft 14 extends and is connected to the orbiting scroll 20 on an opposing side of the fixed scroll to the motor 18. A high vacuum region 30 is located at the inlet 24 and a low vacuum, or atmospheric, region 32 is located at the outlet 26.
A counter-weight 44 balances the weight of the orbiting components of the pump, including the orbiting scroll 20, the second bearing 36 and the eccentric portion 16 of the drive shaft. The orbiting scroll 20 constitutes the majority of the weight of the orbiting components and its centre of mass is located relatively close to the scroll plate of the orbiting scroll. A cap 46 is fixed to a raised seat 48 of the orbiting scroll and seals low vacuum region, containing the counter-weight and the bearings 34, 36 from the high vacuum region 30.
An anti-rotation device 50 is located in the high vacuum region 30 of the pump and is connected to the orbiting scroll 20 and the housing 12. The anti-rotation device resists rotation of the orbiting scroll but allows orbiting motion of the orbiting scroll. The anti-rotation device is lubricant free and in this example is made from a plastics material, and may be a one-piece polymer component as described in greater detail in the earlier application.
A first bearing 34 supports the concentric portion of the drive shaft 14 for rotation. The bearing 34 is fixed relative to the housing or as shown the fixed scroll 22. A second bearing 36 connects the eccentric portion 16 of the drive shaft to the orbiting scroll 20 allowing angular movement of the orbiting scroll relative to the eccentric portion. A first shaft seal 38 is located between the fixed scroll 22 and the concentric portion 14 of the shaft resists the passage of lubricant from first bearing 34 and gas from the atmospheric side of the pump towards the low pressure side of the pump or into the flow path between the inlet and outlet. A second shaft seal 42 is located between the orbiting scroll 20 and the eccentric portion 16 of the shaft and resists the passage of lubricant from second bearing 36 into the flow path between the inlet and outlet.
Generally there is a desire to produce smaller pumps. The inverted scroll pump provides a more compact solution compared to a non-inverted scroll pump. In the inverted solution the shaft seals described above are used to seal between the shaft and the orbiting scroll and the shaft and the fixed scroll. Scroll pumps are typically caused to rotate at about 1500 rpm but as pumps become smaller there is a requirement to rotate the drive shaft more quickly at speeds of for example 1800 rpm to maintain similar pumping performance. Generally, the shaft seals wear quite quickly and require regular replacement and this problem is exacerbated at higher speeds. A harder seal could be used and may last longer but will seal less effectively.